# mxa — usage

Read, write, and subscribe to AVEVA System Platform tags via MxAccess. The CLI runs in-process: each invocation registers an `LMXProxyServer`, executes, and unregisters cleanly. Errors carry the underlying `MxStatusCategory` so an agent can decide whether the failure is transient (Pending), configurational, or operational.

## Common notes

- **Tag references** are full attribute paths: `<ObjectName>.<AttributeName>` (e.g. `TestMachine_001.Speed`). For `Galaxy:` references, follow the convention used in InTouch / Object Viewer.
- **`--client <name>`** sets the client name passed to MxAccess `Register()`. Defaults to `mxa`. Most install logs key on this string.
- **Timeouts are per-call.** They control how long the CLI waits for a `OnDataChange` (read) or `OnWriteComplete` (write). The default is 5 seconds.
- **First-event latency.** LMX has to resolve the reference and bind to the hosting engine on each fresh client connection. Empirically the first `OnDataChange` arrives **3-8 seconds** after `Advise()`. Set timeouts and `subscribe --seconds` accordingly: a 3-second `read` may legitimately time out on first contact, then succeed on the next try because LMX has cached the binding.
- **Subsequent events are fast.** Once a tag is bound, value-change updates propagate within ~100 ms.
- **Exit codes:** `0` on success, `1` if any operation timed out or returned a non-Ok / non-Pending `MxStatusCategory`, `2` on argument-validation errors.

## `mxa info`

Print the loaded `ArchestrA.MxAccess` assembly identity, supported `--type` values, and the full `MxStatusCategory` enum. No tag access.

```powershell
mxa info
```

## `mxa read <tag> [<tag>...]`

Reads one or more tags by briefly subscribing and capturing the first `OnDataChange` per tag.

| Option | Default | Notes |
| --- | --- | --- |
| `-t`, `--timeout <seconds>` | `5` | Per-tag timeout. Tags that don't deliver a `DataChange` within the window are reported with `error: timeout`. |
| `--client <name>` | `mxa` | Passed to `Register()`. |
| `--llm-json` | off | Emit the JSON envelope. |

Examples:

```powershell
mxa read TestMachine_001.Speed
mxa read TestMachine_001.Speed Reactor1.Level -t 3
mxa read TestMachine_001.Speed Reactor1.Level --llm-json
```

LLM-JSON envelope:

```json
{
  "query":   { "command": "read", "tags": ["TestMachine_001.Speed"], "timeout_s": 5.0, "client": "mxa" },
  "ok":      true,
  "results": [
    {
      "tag":       "TestMachine_001.Speed",
      "ok":        true,
      "value":     1234.5,
      "quality":   192,
      "timestamp": "2026-05-03T19:42:18.001",
      "statuses":  [
        { "Success": 0, "Category": "MxCategoryOk", "DetectedBy": "MxSourceRespondingAutomationObject", "Detail": 0 }
      ]
    }
  ]
}
```

## `mxa write <tag> <value>`

Writes one value to one tag and waits for `OnWriteComplete`.

| Option | Default | Notes |
| --- | --- | --- |
| `--type <kind>` | inferred | Force the .NET type used for the boxed value. One of `bool`, `byte`, `short`, `int`, `long`, `float`, `double`, `string`, `datetime`. |
| `-t`, `--timeout <seconds>` | `5` | How long to wait for `OnWriteComplete`. |
| `--user-id <int>` | `0` | Authenticated user id. `0` is unauthenticated; secured attributes will reject. |
| `--client <name>` | `mxa` | Passed to `Register()`. |
| `--llm-json` | off | Emit the JSON envelope. |

Type inference rules (when `--type` is not set): `true`/`false`/`yes`/`no`/`on`/`off`/`1`/`0` → bool; pure integer → `int` (then `long`); decimals → `double`; everything else → `string`.

Examples:

```powershell
mxa write TestMachine_001.Setpoint 42.5 --type double
mxa write TestMachine_001.RunFlag true
mxa write TestMachine_001.Label   "Hello world"
mxa write Reactor1.Setpoint       100 --type int -t 10 --llm-json
```

The same JSON envelope shape as `read`, with `results[0]` containing `{ tag, ok, error?, statuses }`. No `value`/`quality`/`timestamp` on the write result — consult a follow-up `mxa read` to confirm.

## `mxa subscribe <tag> [<tag>...]`

Streams `OnDataChange` events for a duration.

| Option | Default | Notes |
| --- | --- | --- |
| `-s`, `--seconds <seconds>` | `10` | Wall-clock duration of the subscription. |
| `--max <int>` | `1000` | Hard cap on emitted events. |
| `--client <name>` | `mxa` | Passed to `Register()`. |
| `--llm-json` | off | **JSON Lines** mode — one JSON object per line, no outer envelope. |

Human output:

```text
[INFO] Subscribed to 1 tag(s). Streaming for 30.0s. Ctrl-C to stop early.
[19:42:18.001] [OK ] TestMachine_001.Speed = 1234.5 (q=192)
[19:42:19.002] [OK ] TestMachine_001.Speed = 1245.7 (q=192)
...
[INFO] 30 event(s) emitted; subscription closed.
```

LLM-JSON output (one event per line, no surrounding `[ ... ]`):

```jsonl
{"tag":"TestMachine_001.Speed","ok":true,"value":1234.5,"quality":192,"timestamp":"2026-05-03T19:42:18.001","statuses":[{...}]}
{"tag":"TestMachine_001.Speed","ok":true,"value":1245.7,"quality":192,"timestamp":"2026-05-03T19:42:19.002","statuses":[{...}]}
```

JSON Lines lets a downstream consumer parse events incrementally rather than buffering the whole stream — the right shape for indefinite or long-running subscriptions.

## Errors and statuses

Every `result` carries a `statuses` array — the elements of the COM `MXSTATUS_PROXY[]` MxAccess passes back. Field names match the C# struct exactly:

| Field | Type | Meaning |
| --- | --- | --- |
| `Success`    | int16 | 0 = Ok, non-zero = error code |
| `Category`   | enum  | `MxCategoryOk`, `MxCategoryPending`, `MxCategoryWarning`, `MxCategoryCommunicationError`, `MxCategoryConfigurationError`, `MxCategoryOperationalError`, `MxCategorySecurityError`, `MxCategorySoftwareError`, `MxCategoryOtherError`, `MxStatusCategoryUnknown` |
| `DetectedBy` | enum  | `MxSourceRequestingLmx`, `MxSourceRespondingLmx`, `MxSourceRequestingNmx`, `MxSourceRespondingNmx`, `MxSourceRequestingAutomationObject`, `MxSourceRespondingAutomationObject`, `MxSourceUnknown` |
| `Detail`     | int16 | Additional error-code detail |

A result is considered `ok` only if every `statuses` element has `Category in (MxCategoryOk, MxCategoryPending)`.

Common failure shapes:

- **`Category: MxCategoryConfigurationError`** — usually a typo'd reference or the attribute doesn't exist on the deployed instance. Sanity-check via `graccesscli object snapshot`.
- **`Category: MxCategoryCommunicationError`** — engine isn't running, object is OffScan, or LMX can't reach the platform hosting the object.
- **`Category: MxCategorySecurityError`** — secured attribute, `--user-id 0`. Use `WriteSecured` semantics (not yet exposed by this CLI) or target a `Writeable_USC_*` attribute.
- **Timeout** — most likely the tag is genuinely silent (no value updates) or the reference is wrong. With `--llm-json` you'll see `"error": "timeout"` and an empty `statuses`.

## Picking a tag for a smoke test

If the live galaxy is not familiar:

1. Connect to the Galaxy Repository SQL — see [`../../grdb/connectioninfo.md`](../../grdb/connectioninfo.md).
2. Find a deployed instance with a writeable UDA — [`../../grdb/queries/attributes.sql`](../../grdb/queries/attributes.sql) lists user-defined attributes with their data type. Filter on a `Writeable_*` security classification (see [`../../aot/dev-guide/appendix-e-security-classifications.md`](../../aot/dev-guide/appendix-e-security-classifications.md)).
3. The reference for MxAccess is `<InstanceName>.<AttributeName>`.